The invention relates to a knee joint prosthesis in accordance with the preamble of the independent claim.
Knee joint prostheses are available today in a large variety of various designs, with their principles of functioning often differing greatly from one another. This is related--among other things--to whether and, where appropriate, which ligaments (e.g. cross ligaments, lateral ligaments) are present and still capable of functioning in the respective patient in whom the prosthesis is to be implanted.
A knee joint prosthesis of this kind is disclosed for example in EP-A-0,724,868. The knee joint prosthesis described there comprises a tibia part which is anchored in the tibia and which has a tibia bearing surface on which a bearing body in the form of a meniscus part is slidingly displaceably arranged. The path along which the meniscus part is guided is limited in several exemplary embodiments (e.g. FIGS. 1a, 1b, 1c) to the anterior/posterior direction. For this purpose the tibia part is provided with a guide part or guide piece which engages in a corresponding guide part (elongate hole or dove-tail groove) in the meniscus part, through which the meniscus part is displaceable only in the anterior/posterior direction.
The guide piece which is provided at the tibia part has a bar-shaped prolongation which points in the proximal direction, thus in the direction of the femur part. This bar-shaped prolongation extends in a direction parallel to the longitudinal axis of the tibia part. A coupling member in the form of a cylindrical bolt which has a passage bore extending perpendicular to the longitudinal axis of the bolt is provided for the coupling of this prolongation and thus of the guide pieces provided on the tibia part to the femur part.
The femur part has two condyles which can move on the corresponding bearing scallops (bearing surfaces) of the bearing body. The femur part further has two side walls, in each of which a cylindrical cut-out is provided which breaks through the side wall.
The above described cylindrical bolt with its passage bore extending perpendicular to the longitudinal axis of the bolt is inserted into these cut-outs in the side walls and the bar shaped prolongation of the guide piece is passed through the cylindrical bore of the bolt. The bolt thus acts as a coupling member because it couples the femur part to the prolongation on the guide piece during the flexion and extension movement of the knee. During the extension and/or flexion of the knee the cylindrical bolt slides upwards (extension) or downwards (flexion) respectively along the prolongation on the guide piece. At the same time the meniscus part is displaced in the anterior (extension) or posterior direction (flexion) respectively.
The knee joint prosthesis described in EP-A-0,724,868 is absolutely capable of functioning from the point of view of its principle. The assembly of the prosthesis in the operating room is not completely simple for the surgeon, however, because the bar-shaped prolongation of the guide piece must be guided through the bore in the cylindrical bolt after the tibia part has been fixed in the tibia and the femur part has been fixed in the femur, thus practically a kind of "threading procedure" must be carried out. This can cause difficulties for the surgeon under certain circumstances since the tibia part and the femur part are of course already fixed in the corresponding bones.
Moreover, the following case, which is described with reference to FIG. 1, can however also arise in the prosthesis described in EP-A-0,724,868. During flexion the cylindrical bolt B moves downwards along the bar shaped prolongation S of the guide piece F while the meniscus part M moves in the posterior direction. The case can now arise that the femur lies in contact with the rear end E of the meniscus part M although a complete bending has not occurred. If one were now to attempt to carry out a further bending, the center of rotation would have to be formed by the rear end E of the meniscus part M. This would mean that the cylindrical bolt B would have to move into the position indicated in broken lines during a further bending. This cannot take place however because the bolt B can of course only be moved upwards and downwards along the prolongation S of the guide piece F (and not on a circular path as it would have to be). The result is a blocking of the prosthesis, which prevents a further flexion. In worse cases, when the forces are sufficiently large, the result can even be a loosening of the femur part of the prosthesis.